Sounds like you need standard height but uprated suspension springs. Example price - around £100 per set of 4 springs from Avo. Others available.

Re the air filter - either fit a cone type (but make sure it fits onto the standard diameter hose), or a K+N panel filter that will fit into the original airbox - either will be OK. Rough prices used cone filter around £40, new cone around £90, new K+N panel filter about £45 off ebay.

Re exhaust - if it's got a stainless on already, it should be fine. If it's not stainless, get one - check ebay. Note - 2wd and 4x4 exhausts are different.